The Phantom of the Institute
by boyslikegirlslikeboys
Summary: Cassidee is struggling to find an outfit for a costume party... T for safeness, JacexCassidee, as usual.


Cassidee stared unhappily at her closet, one hand on her hip and a small frown on her lips. Nothing she had would work. Absolutely nothing. Not one single scrap of clothing. With a huff, she sat down on her bed and pulled out her cell.

Magnus answered on the first ring.

"I had a feeling I'd be hearing from you," He said, a hint of laughter in his voice. "I'm coming over right now."

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"So what look are you going for?" He asked her, sprawled out on her bed. Cassidee shrugged, giving another dismayed glance at her clothes.

"I don't have any idea."

"That's pretty funny, considering this whole party thing was _your_ idea." Cassidee grimaced, knowing he was right. She hadn't really thought anything would come of it, but now that people had taken to the notion, she couldn't back out. _A costume party, _she'd excitedly announced to her friends, _Just like Halloween! _Her lips twisted in a wry smile at Magnus. "Alright, well, let me think. You could be a vampire."

"And risk getting attacked by werewolves at the party? Not a chance."

"What about an Oompa-Loompa? You're short enough," Magnus laughed. Cassidee stuck out her tongue at him. "Maybe you could go as a Shadowhunter!"

"Jace said I wasn't vicious enough to pass for a Shadowhunter." Cassidee remembered his exact words: _Let's just face it, baby. If I'm a lion, and I'm a Shadowhunter… well, you're a bunny. Or a mouse. Or some other small, defenseless animal with a soft body and no predatory instinct._ She rolled her eyes.

"Well, you should be a princess. Like the ones in those animated movies you watch all the time; that one with the hairy monster and the girl who likes to read, that would be good."

"But princesses are so ordinary!" Cassidee whined. Magnus released a groan, catlike eyes scanning Cassidee's room for inspiration. Suddenly, he stood up and strode across the floor, stooping to pick up a movie case. He was grinning, satisfied with his solution. Cassidee read the movie's title, gasped, and nodded with crazy enthusiasm.

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"Wow. I mean, I usually look amazing, but tonight I am _exceptionally_ gorgeous." Isabelle preened in the reflective sides of the Institute's elevator. She wore a miniscule maid's outfit, complete with a frilly white garter around her left thigh. Her dark Marks swirled around her skin, somehow adding to the costume. Alec sighed.

"Modesty just isn't your strong suit, is it?" He asked, sticking a hand inside his pocket. After threats and fights, Isabelle had managed to force her brother to wear an old mechanic's uniform. In her opinion, it was a pitiful excuse for a costume, but to Alec it was over-the-top.

"Not in the least." Isabelle fluffed her hair one last time before stepping out of the elevator and into the foyer of the Institute. "Alright, we'd better get to the park. Jace is probably wondering where we are."

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Jace adjusted his mask for the umpteenth time, unsure about what to do. Music already thumped across the expanse of the park, and costumed Downworlders had been showing up for an hour. He was relieved to see Isabelle and Alec coming into the party; he wove his way through the crowd to meet them.

"Whoa, Jace, nice costume," Izzy said appreciatively. Jace nodded distractedly, searching for a familiar head of blond hair. "She's not with us. She said her costume was supposed to be a surprise for everyone." Alec's phone buzzed, signaling a text. He flipped it out and read it.

"Cassidee's with Magnus, they're on their way here."

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As the taxi pulled up to the park, Cassidee was pleased to see that all the Downworlders were getting along just fine. She had been expecting fights and mayhem, but it was a welcome surprise to find that everyone was dancing and enjoying the night. Magnus shot her a big grin and adjusted his hat. He'd elected to dress as a very glittery Peter Pan.

"Ready?" He asked. She nodded, checking to see that her costume was all set. Magnus opened the door, waving to Alec, Isabelle, and Jace, who were headed for the taxi. Cassidee still sat in the backseat, waiting for the grand reveal that Magnus had planned for her.

"Are you wearing tights?" Alec asked, a bit of alarm in his voice. Magnus laughed.

"What's a Peter Pan costume without tights?"

"Straight," Jace replied sarcastically. Cassidee giggled under her breath, gaining Jace's attention. "Alright, Cass, let's see it." Taking a deep breath, Cassidee prepared for her entrance.

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Jace watched as a bare foot snaked its way out of the taxi, followed by an ankle and a slender calf. A golden skirt flowed down Cassidee's legs as she stood, with a sash tied diagonally across her waist. Her torso was covered with a white off-the-shoulder blouse, which in turn had a dark brown corset over it. The blond strands of her hair were curled loosely into waves, a red rose tucked behind her ear. She looked up at Jace… and began to giggle.

Black pants, black boots, a black shirt—normal attire for Jace. However, this time it wasn't Shadowhunter gear; it was the clothing for a gentleman. Over his shoulders, a black cape was draped, stirring slightly in the breeze.

Half of his face was obscured by a white mask.

"Miss Cassidee," Jace said with a crooked smirk, sweeping his cape behind him in a bow. Cassidee curtsied, smiling at him.

"The Phantom of the Institute," She deemed him happily. He wrapped an arm around her waist, while she snuggled into the warmth of his body. Together, they made their way down to the party.

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"It's almost creepy," Alec said a little while later, watching the couple on the dance floor. Jace was holding Cassidee delicately, as if she were a mirage that he was afraid to disturb. His golden eyes were soft, the ghost of a smile on his lips. Cassidee had her arms looped around his neck, fingers stroking the silky golden hairs that were within her reach. While Alec observed, she rested her cheek against his chest lightly, and Jace leaned down to plant a kiss on the top of her head.

"What's creepy about it?" Magnus asked, curious. Alec gestured to them.

"They managed to dress the same, even without coordinating it. It's like they're made for each other. Like two halves of one person."

"Why's that creepy?" The more Alec thought about Magnus's question, the farther from an answer he was. Jace seemed more peaceful when Cassidee was around, and Cassidee was less animated without Jace. Maybe two halves of one whole wasn't such a bad thing as Alec had thought. In a way, one of them wouldn't be complete without the other one. Like Jace, the Phantom of the Institute, without his mask.

"Well… maybe it isn't."

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**A/N: I really need to chill with the Mortal Instruments fics, but Jace is sooooooo fun to write about! Anyway, this is probably pretty poo-y, but that's okay; I just wanted to updateeee. Review, rate, whatever ya'll wanna do! **


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